Brittle stars, also known as serpent stars or ophiuroids (scientific name Ophiura ophiura), are echinoderms closely related to starfish. With over 2000 known species, these fascinating creatures exhibit diverse feeding habits and play crucial roles in marine ecosystems. This article delves into the brittle star's diet, feeding mechanisms, and ecological significance.
Brittle Star Taxonomy and Morphology
Belonging to the class Ophiuroidea and the genus Ophiothrix, brittle stars are characterized by a central disc and five to six flexible arms used for locomotion. The genus Ophiothrix is the widest and is found worldwide in almost all oceans stretching from the Arctic to the Antarctic region. The genus Ophiothrix has been subdivided into three important subgenera that include a few hundred of the species of the brittle star; The subgenera are Theophiothrix, Acanthophiothrix and Ophiothrix. These arms are long, slender, and whip-like, capable of growing up to 60 cm (24 inches) in the largest specimens.
Unlike sea stars, brittle stars lack suction cups on their tube feet, which are primarily used for feeding and sensory perception. The water vascular system, common to echinoderms, controls locomotion, respiration, digestion, and waste transportation. A madreporite, located on the ventral surface, regulates water flow into and out of the body.
The central disc houses the brittle star's organs, including a large stomach, genitals, muscles, and mouth. Notably, brittle stars lack brains and eyes, relying on sensory nerves in their epidermis to perceive their environment.
Dietary Habits: Scavengers, Detritivores, and Predators
Brittle stars exhibit a range of feeding strategies, primarily acting as scavengers and detritivores. In reef aquariums, they roam the substrate and rockwork, consuming tiny particulate matter and leftover fish food. Tube feet transport these meals to the mouth.
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- Scavenging and Detritivory: Most brittle stars primarily feed on detritus and tiny particulate matter.
- Predation: Some serpent starfish species are known to prey on small organisms like worms, such as feather duster tube worms. Certain species raise their arms to trap prey, wrapping them up and eating them.
Some aquarists have observed their brittle stars consuming snails. While most brittle stars are primarily scavengers, some, like the green brittle star, are opportunistic predators. The anecdotal evidence suggests that individual behavior, tank conditions, and feeding frequency influence predatory behavior.
Feeding Mechanisms: A Closer Look
Brittle stars utilize various mechanisms to acquire food:
- Suspension Feeding: Some species are passive suspension feeders, using papillae to consume food and phytoplankton.
- Deposit Feeding: Brittle stars ingest organic material from the substrate.
- Target Feeding: Some aquarists spot-feed their brittle stars with large chunks of food, such as squid, to ensure their health.
The brittle star raises its arms to capture tiny living organisms and sea algae. Mucous strands inside the tube feet trap the food, which is then swept into the mouth. The food travels through the esophagus to the stomach, which has almost 10 pouches for digestion. Since brittle stars lack an anus, waste is expelled through the mouth.
Habitat and Distribution: From Reefs to the Deep Sea
Brittle stars have a widespread distribution, inhabiting oceans from the poles to the tropics. They are commonly found in coral reefs, where they hide within rock hollows or among living organisms. While some species tolerate brackish water with high salinity, most dwell on the sea beds at depths exceeding 6000 meters. Basket stars, another type of ophiuroid, are typically found in the deep regions of the ocean.
Reproduction and Life Cycle
Brittle stars reproduce both sexually and asexually. Sexual reproduction involves the release of eggs and sperm into the water for external fertilization. Some species, like Amphipholis Squamata (small brittle stars), brood their eggs near the base of their arms in cilia-lined sacs called bursae. Asexual reproduction occurs through fission, where the central disc splits in half, and each half regenerates into a new individual. Brittle stars typically live for around 5 years, reaching sexual maturity at 2 years old and full growth by 3 or 4 years old.
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Defense Mechanisms: Autotomy
Brittle stars possess a unique defense mechanism called autotomy, or self-amputation. When threatened by predators, they can detach one or more arms from the central disc. This process is facilitated by mutable collagenous tissues (MCT) at the base of the arms, which disintegrate, causing the arm to fall off. The lost arm can regenerate over weeks or months, depending on the species.
Locomotion and Movement
Despite having a radially symmetrical body, brittle stars do not use their tube feet for locomotion. Instead, they use their flexible arms to wriggle through the water. This movement is coordinated by the arms on either side of the direction of travel, resembling the flipping movement of a sea turtle swimming.
Ecological Roles and Significance
Brittle stars play several important ecological roles in marine ecosystems:
- Scavengers: They consume detritus and organic matter, helping to keep the ocean floor clean.
- Predators: Some species prey on small invertebrates, regulating their populations.
- Food Source: Brittle stars serve as a food source for various predators, including starfish, velvet crabs, and spiny starfish.
Brittle Stars in Reef Aquariums
In reef aquariums, brittle stars are often kept as part of the cleanup crew. They help to control detritus and uneaten food, contributing to a healthy environment. However, it is important to consider their potential predatory behavior, especially with smaller fish and invertebrates.
Conservation Status
Common brittle stars have not been evaluated for the IUCN Redlist.
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